Corn (Zea mays L.) is one of the most important commercial crops in the world. Like many commercial crops, corn is subjected to numerous potentially detrimental environmental conditions (e.g., moisture availability, temperature stresses, soil conditions, pests, disease) that can reduce, or entirely eliminate, crop yield. Crop disease alone accounted for the loss of more than 1.3 billion bushels of corn in the United States and Ontario, Canada in 2012. See Mueller, Corn Disease Loss Estimates from the United States and Ontario, Canada—2012. Purdue Extension Publication BP-96-12--W (2014).
Northern leaf blight (NLB) is a crop disease caused by the fungal pathogen Exserohilum turcicum (also referred to as Helminthosporium turcicum or Setosphaeria turcica in some literature). NLB can infect corn in tropical and temperate climates. Infected corn plants can exhibit a range of symptoms from cigar-shaped lesions on lower leaves to complete destruction of multiple leaves. Corn infected with NLB is also highly susceptible to stem rot and root rot caused by secondary infections. NLB is particularly problematic in tropical highlands, where environmental conditions favor disease development. However, NLB infection can cause yield losses of 30%-50% in temperate environments, including the United States and Europe.
E. turcicum overwinters as mycelia and conidia on corn plant parts left on the soil surface. The conidia are transformed into spores, and during warm, wet weather, new conidia are produced and carried by wind or rain onto the lower leaves of corn plants. Infection requires the presence of water on the leaf surface for at least 6 hours and a temperature of between 65° F. and 80° F. If infection occurs, lesions develop within 12 days and produce new conidia which can spread the infection to additional leaves and plants. NLB management strategies include crop rotation, destruction of over-wintering corn plant parts, and fungicide application. However, fungicide application alone is not an efficient mechanism of control, especially in Brazil.
There is a need in corn breeding to identify corn germplasm that provides resistance to NLB infection. There is also a need to develop polymorphic markers for monitoring and introgressing NLB resistance alleles, and further develop agronomically elite corn lines comprising NLB resistance for enhancing plant productivity.